1. Field of the Invention
The present invention relates to rotation rate sensors more particularly, this invention pertains to a multi-axis fiber optic rotation rate sensor.
2. Description of the Prior Art
A minimal configuration of a single-axis fiber optic rotational speed sensor (refer to FIG. 1) comprises a light source LQ, a first beam splitter ST.sub.1, a polarizer P, a second beam splitter ST.sub.2, a phase modulator PM and a connected fiber coil F combined in a dashed border MCS (Minimal Configuration Single-axis) forming a circuit arrangement (circuit block). The returning light beams, affected by the fiber coil, are coupled out at a second output of the first beam splitter ST.sub.1 and pass to a photodetector PD whose electrical signals are applied to an A/D converter AD that feeds a known digital signal readout, and a closed loop CL for resetting a (possible) Sagnac phase. The reset signal is applied via a D/A converter DA to the phase modulator PM in a known arrangement.
It has been proposed in European patent application Serial No. EP-A1-0,400,197 to operate the light source LQ in a pulsed fashion to simplify evaluation of the readout signal and permit the use of low-cost converter modules and microprocessors. More particularly, that patent application proposes operating LQ so that the operating time corresponds precisely to the transit time of light through the fiber coil F. This enables one to transform the output signal at the photodetector PD in a low-frequency fashion so that further processing can take place at frequency bands that are readily controlled.
The measurement of a plurality (in particular, three) independent rotation rates by means of the above described minimal configuration of FIG. 1 has required a corresponding plurality of independent fiber gyro structures. This is unfortunate as the subassemblies or modules bounded by the circuit block MCS of FIG. 1 contain, at least in part, very expensive components. For example, when a superluminescent diode is used for the light source LQ, a Peltier element having an assigned control circuit is required for temperature stabilization.
U.S. patent application Ser. No. 4,588,296 (refer particularly to FIG. 40B and accompanying description) discloses a three-axis fiber-optic rotational rate sensor in which, after passing through a space filter, the light is split into three identical beam pairs by a beam-splitter module fabricated in integrated optics. The beams are radiated into three identical fiber coils aligned along the three spatial axes. The three readout signals may be separated and distinguished by multiplex selection and evaluation. This triad arrangement is affected by a fundamental technological problem resulting from the fact that exceptionally high frequencies must be processed during readout and in the reset electronics. This problem becomes more acute when the lengths of the individual fiber coils are shortened.